Various lasers have been developed for engraving, cutting, welding, annealing, etc. various materials. Lasers have also been developed or proposed for delivering laser energy to a site or target on or in a mammalian body for diagnostic or therapeutic purposes. These lasers typically deliver laser energy to a target site either directly or through delivery devices such as an articulated arm, a hollow wave-guide or a flexible optical fiber. If pulsed laser energy is desired in these applications, it is usually provided in a train of evenly spaced pulses.
In therapeutic veterinary or medical applications, laser energy is used to produce a desired effect on various types of tissue. The laser energy interacts with the tissue through reflection, refraction or absorption. This interaction may be used to perform incision, excision, resection, vaporization, ablation, coagulation, hemostasis, and denaturization of various tissues. One or more of the above effects of laser radiation on tissue may be produced with pulses of laser radiation having, for example, a wavelength of about 2,100 nm, a pulse width of about 300 microseconds and an energy density of about 1 J/cm2 incident on the target site. Laser energy of this wavelength is highly absorbed by water, a constituent of virtually all tissues.
The effect of laser radiation on tissue is dependent upon several factors, some of which include the type of tissue irradiated, the amount of radiation absorbed by the tissue, the time during which the laser energy is delivered and the absorption efficiency of the wavelength of laser energy on the target tissue. Relatively hard or dense tissues, such as calcified tissues or bone which may have a comparatively low water content, require relatively high energy levels for effective ablation. For example, at a wavelength of about 2,100 nm, this would optimally require an energy density of 1 to 20,000 J/cm at a pulse width of 100 to 800 μs at the target tissue site.
In a variety of applications, including surgical procedures, where ablation, vaporization or other effects are desired, it is preferred to achieve these effects relatively quickly to reduce thermal conduction and damage to nearby tissues. Also, in these applications, it may be desirable to increase the time period between pulses, to allow additional time for the target to cool between pulses. In order to ablate certain types of tissue quickly, the laser radiation incident at the site or target of application, for example, at a wavelength of about 2,100 nm, should preferably be delivered in pulses of 1 to 10 Joules of energy with a pulse width of 100 to 800 μs at a repetition rate of 1 to 100 Hz.
The production of such high energy levels with a single laser resonator or oscillator (e.g. a source of high intensity optical radiation such as from a flash lamp, arc lamp or diode-laser, and a lasing medium) is difficult or impossible. This is especially true for thulium holmium:YAG, chromium thulium holmium:YAG, erbium:YAG, thulium:YAG, ruby or similar lasers having a limited energy output capability. Many commercially available lasers that are suitable for ablation or vaporization of tissue cannot be operated for extended periods of time at such high energy levels, without creating excessive heat or placing excessive stress on the laser system and/or an optional optical delivery mechanism, which can lead to premature component failure.
Other methods of generating multiple consecutive laser pulses within a short period of time are known in the art and have previously been disclosed. However, the implementation of these methods requires an increased number of components, complexity, and cost compared to the present invention. Additionally, these other methods typically require more input power than the present invention in order to achieve the same target effects.
Accordingly, it would be desirable to provide an improved laser system capable of generating radiant energy at higher effective energy levels to the target site. Preferably, such an improved system should accommodate the use of commercially available, pulsed lasers of the following types: erbium:yttrium aluminum garnet (Er:YAG), thulium:yttrium aluminum garnet (Tm:YAG), thulium holmium:yttrium aluminum garnet (TmHo:YAG), chromium thulium holmium:yttrium aluminum garnet (CrTmHo:YAG), neodymium:yttrium aluminum garnet (Nd:YAG), alexandrite, ruby and other pulsed lasers.
Desirably, such an improved laser system should deliver radiant energy to a target with a relatively long thermal diffusion time or relatively low thermal conductivity (e.g. tissue, bone, hair, cotton, plastic, wood, etc.) in a pulse train that has a sufficiently high energy level during a relatively short time period in order to quickly raise its temperature to produce the desired effect on the target, while lengthening the time between pulses to allow additional time for the target to cool between pulses.
Conceptually, if this rapid target temperature rise is produced using a series or train of evenly spaced pulses of laser energy, the temperature of the target (e.g. bone, organs, cartilage, etc.) will start to decay back to its ambient temperature after the end of each pulse in this train. It is understood that the temperature of a target that has been raised above its ambient temperature Ta to an elevated temperature Ts after the end of each pulse in this train decreases ideally according to the following equation:Te=Ta+(Ts−Ta)e−t/kwhere Ts is the maximum elevated temperature to which the tissue has been raised by a preceding pulse or pulses, e is the natural logarithm base, t is any selected time period following the achievement of temperature Ts, k is the target thermal diffusion time constant, and Te is the resulting time-dependent temperature at the end of time period t.
When a target (e.g. tissue) is subjected to a pulse of laser energy, the target temperature rises to a maximum temperature Ts, and then begins to decrease. If the maximum target temperature after the end of a pulse of laser energy Ts were below the desired target temperature Td, it would be desirable to provide increased energy to the target to allow the desired target temperature Td to be achieved. It is believed that the efficiency of laser effects (e.g. vaporization or ablation) on targets with a relatively long thermal diffusion time can be increased by subjecting the target to pulses of laser energy in a way that results in little or no temperature decay between laser pulses. Accordingly, to achieve this increased efficiency, the time span between consecutive laser pulses in the pulse train should be relatively short, preferably much shorter than the target thermal diffusion time constant.
For example, when a target site of a typical human tissue, such as muscle or cartilage, is elevated to an initial temperature of about 120° C., the tissue temperature decays to 115° C. in about 10 milliseconds. It would be desirable to subject the tissue to a plurality of laser pulses in less than or equal to that time period. A preferred laser system for the ablation or vaporization of such tissue should accommodate the emission of two or more laser pulses with a typical temporal separation of less than 10 milliseconds between the pulses, with a pulse separation time and pulse width that depend upon the desired peak pulse energy and target effects; for example, a pulse separation time of 1 ms and a pulse width of 100 to 800 μs at a wavelength of about 2,010 nm. The pulse separation and/or pulse width may vary significantly depending upon the specific application. For example, in order to ablate or fragment bladder, kidney, or ureteral stones, a pulse separation of 10 μs and a pulse width of 1-10,000 nanoseconds may be desirable.
As is previously known from many literature sources, many solid-state lasing ions exist, of which many are bivalent and trivalent lanthanides, for example, praseodymium (Pr3+), neodymium (Nd3+), samarium (Sm2+), europium (Eu3+), gadolinium (Gd3+), terbium (Tb3+), dysprosium (Dy2+), holmium (Ho3+), erbium (Er3+), thulium (Tm2+, Tm3+) and ytterbium (Yb3+). Other solid-state lasing ions are also well known, for example, titanium (Ti3+), vanadium (V2+), chromium (Cr2+, Cr3+ and Cr4+) and others.
At, above, and/or below room temperature, there are many different host crystals that may be used in conjunction with many of the above solid-state lasing ions or combinations thereof, including, for example, Y3Al5O12 (YAG), Y3Sc2Ga3O12 (YSGG), LiYF4 (YLF), Gd3Sc2Ga3O12 (GSGG), Y3Ga5O12 (YGG), Y3AlO3 (YAP), LaF3, BaY2F8, KCaF3 and others. Other solid-state host materials, such as plastics or gelatins, may also be used in conjunction with many solid-state lasing ions. The selection of a relatively transparent host material that is sensitized or doped with various relative percentage(s) of one or more lasing ion(s) determines the wavelength and other properties of the solid-state active lasing medium.
One or more lasing-related properties may be used to classify various solid-state lasing media. A subset of all available solid-state lasing media exhibit the characteristics of relatively long emission and energy storage lifetimes, for example, on the order of 100 μs or longer. Many of these types of solid-state lasing media are well known, for example, Er:YAG, Tm:YAG, Ho:YAG, Er:YSGG, Tm:YSGG, TmHo:YAG, CrTmHo:YAG, erbium-doped fiber amplifiers and others, and have been characterized as to both the predicted and actual characteristics of emission and energy storage lifetimes. The emission lifetime of these media are usually longer than the energy storage lifetime due to, for example, impurities in the lasing medium, constructional constraints imposed by the laser resonator design, sub-optimal thermal management and other factors. It would be desirable to utilize these properties to produce advantageous pumping and energy extraction and deliver a plurality of laser pulses within a short time period.
The present invention provides an improved laser energy generation system that can produce the above-discussed benefits and features.